drm/i915: move ibx_digital_port_connected to intel_dp.c

The function can be made static there. No functional changes.

Reviewed-by: Durgadoss R <durgadoss.r@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>